Structstore

Support defining custom structs

This also introduces a CRTP field base class and brings some small refactorings and simplifications in the type registry and build scripts

This improves on the type registry, Python bindings, logging, and test coverage

Implement generalized typing backend

This implements and switches to a generalizing typing backend to support more and also user-defined types. This merge also adds CI tests.

improved operator[] and at() handling

improved move and exception handling

fix deepcopy for matrix fields